Western governments are working to split Zimbabwe's ruling Zanu-PF from President Robert Mugabe ahead of a potentially decisive meeting this week.

Diplomatic sources say Britain and the US believe that the strongest challenge to Mr Mugabe comes not from the opposition but from within the ruling Zanu-PF and they are encouraging dissent by reassuring rebellious factions that their problem is with Zimbabwe's president not the ruling party.

Western officials are looking in particular to the former army chief, Solomon Mujuru, who is seeking to curtail Mr Mugabe's rule at a Zanu-PF central committee meeting on Thursday.

Mr Mugabe has already conceded defeat in his attempt to delay the next election until 2010 and now faces a fight to get Zanu-PF, which increasingly fears heavy defeat in a free election, to adopt him as its candidate in next year's presidential election.

Mr Mujuru's emissaries have been in talks with the main faction of the opposition Movement for Democratic Change led by Morgan Tsvangirai, who is strongly backed by the UK, over the creation of an interim power-sharing government that would sideline Mr Mugabe.

Under such an agreement, Zanu-PF leaders, including Mr Mugabe, would be granted amnesty from prosecution for past crimes such as the Matabeleland massacres in the 1980s and more recent violence. Mr Tsvangirai would call for an immediate resumption of aid to revive Zimbabwe's crumbling economy.

Mr Mujuru has met European and US officials who have said such an agreement would end targeted sanctions against Zanu-PF officials, including travel restrictions, and lead to a resumption of aid.

South Africa is also being brought on board as a potential broker. Mr Mujuru's wife, Joice, who is Zimbabwe's vice-president and now hostile to Mr Mugabe, met South Africa's deputy president, Phumzile Mlambo-Ngcuka, in Johannesburg on Friday. Although billed as a private discussion, that it took place at all reflects a shift away from Pretoria's previous determination not to upset Mr Mugabe.

Mr Mujuru is likely to win support for blocking another presidential term for Mr Mugabe from a rival Zanu-PF faction led by a former state security chief and cabinet minister, Emmerson Mnangagwa. However, there is apparently no agreement between the two groups on who should replace Mr Mugabe as president, which could make it difficult to press the plan forward.

The diplomatic moves are a reflection of the frustration in Whitehall and Washington at the vacuum left by a weak and divided political opposition.

"There is not much confidence in the opposition. It has let the people down at every turn. There's not much prospect of them winning an election while Mugabe's in power and fixing it," said a western diplomat.

"The feeling is that the way forward is a deal between those in Zanu-PF that want rid of Mugabe to try and save their party and those in the opposition prepared to work with them. It's the best way of bringing about swift change and if they can come to a deal, that changes everything. That is what we are working toward."

But some of Mr Mugabe's other political opponents remain suspicious of Mr Mujuru. They believe that as one of Zimbabwe's richest people he is primarily interested in protecting his assets by ensuring that Zanu-PF does not fall with Mr Mugabe.

Arthur Mutambara, leader of the MDC faction that broke away from Mr Tsvangirai and has the support of about half the party's MPs, said that while he favoured a power-sharing transitional government he would not accept one based solely on an agreement between Zanu-PF and Mr Tsvangirai.

"We don't want that kind of nonsense. We want constitutional reform before free and fair elections. We don't want opportunistic alliances that don't provide a long term solution," Mr Mutambara said.

Foreign involvement has infuriated Mr Mugabe who reportedly views Mr Mujuru as a British stooge because he has a financial interest in a UK firm which lost diamond mining concessions in Zimbabwe.

Mr Mugabe has also accused Mr Tsvangirai of being an agent of the British. "Tsvangirai, you want to rule this country on behalf of Blair," he told supporters. "As long as I am alive that will never happen."

But western governments are in the mood for confrontation. After Zimbabwe threatened to expel diplomats that involve themselves in local politics, the US ambassador in Harare, Christopher Dell, gave a press interview in which he said Zanu-PF members increasingly want Mr Mugabe to go and that there is growing dissent within the army and police. He said the country had reached a "tipping point" because of a "new spirit of resistance, some would say defiance, on the part of the people".